Pressure sensitive adhesives (PSA) and tapes made therewith, are used in a variety of applications that expose the PSA and/or the tape to temperatures of 100° C. or higher. For example, PSAs are used to bond printed circuits to heat sinks or rigidizers in many types of automotive and industrial electronics applications such as automobile control modules and other consumer electronics. Another high temperature use for PSAs is in bonding ceramic cook tops to the coated (e.g., painted) metal on stoves. The aforementioned heat sinks are usually made of metals, while rigidizers are normally made of plastics or metals. PSAs are also used to bond ceramic chip components to heat sinks in many electronic devices. In such a use, the tape may be exposed to solder reflow temperatures that are at least 35 to 45° C. above a eutectic Sn/Pb solder liquidus, and these temperatures may be as high as 230° C. to 250° C. where no—Pb (lead free) solder alloys are used. Other examples include use of PSAs in the manufacture of silicon wafers and in the assembly of chip scale packages (CSP). These applications require high temperature, heat tolerant, PSAs that will adhere thin, smooth, silicon wafer surfaces to sapphire surfaces, and will bond chips to flex circuits or adhere one chip to another chip to complete the CSP assembly.
High temperature, or heat tolerant, PSAs and tapes will exhibit good adhesion to a substrate at high temperatures and will permit only low levels of gas generation, or “outgassing.” For example, in the solder reflow processing of flex circuits and in the bonding of heat sinks for automotive engine control modules, the PSA must provide an adhesive bond that is strong enough to avoid adhesive delamination at high temperatures for short periods of time. The automotive industry presently employs electrical flex circuits having power transistor components (semiconductors) that are rated to 175° C., i.e., the actual temperature of operation for such semiconductor power transistors may be 175° C. for extended periods of time during normal car operation. Thus PSAs used to bond such flex circuits to heat sinks should provide stable adhesive properties for extended periods of time at temperatures of 175° C.
Contamination can be a problem, particularly in electronics applications, because a contaminant can interfere with subsequent processing steps, and the outgassing of organic or ionic residual components can contaminate a surface and may cause corrosion. The outgassed components can potentially affect the performance or reliability of an electronic device. Sources of these outgassed organic components include unreacted monomer, monomer impurities, and various low molecular weight organic compounds. Sources of ionic residual components include polymerization initiators, polymerization catalysts, and salts used to control certain chemical processes. Industry specifications include ASTM E-595-93 “Standard Test Method for Total Mass Loss and Collected Volatile Condensable Materials From Outgassing in a Vacuum Environment”, NASA SP-R-0022 [125° C., 24 hrs, in 2×10 exp.−6 Torr vacuum; Total Mass Loss 0.7% and Collected Volatile Condensable Materials 0.01%)], NASA 1224, and Military Specifications 883/5011 (Ionic contaminants) and 883/1018 (Volatile outgassing).
Because of their thermal stability, silicone-based PSAs are currently used in high temperature applications. However, silicone-based adhesives and tapes are expensive and often do not provide sufficient adhesion at temperatures higher than 150° C. Moreover, they tend to leave adhesive residue following removal, and the residue must be removed from the surface in a separate cleaning step prior to further processing.
Acrylic based PSAs are typically prepared from isooctyl acrylate or 2-ethylhexyl acrylate. These adhesives have many desirable attributes such as high peel adhesion when applied to a wide variety of surfaces. Acrylic PSA's, however, do not typically provide high thermal stability and will slowly degrade upon exposure to higher temperature (e.g., above 125° C.). Thermal degradation of these known acrylic adhesives at higher temperatures reduces the cohesive strength of the adhesive and may generate bubble formation from high levels of outgassing, resulting in a loss of adhesion. It is desirable to provide silicone-free PSA's that will strongly adhere to surfaces at temperatures up to at least about 175° C. or even higher temperatures.
The adhesives and articles described herein provide the requisite heat resistance and low outgassing necessary for adhering an article to a surface at high temperatures.